As described in Patent Documents 1 and 2, a photosensitive resin composition for flexographic printing generally contains a thermoplastic elastomer, a photopolymerizable unsaturated monomer, a plasticizer, and a photopolymerization initiator. A structure for a flexographic printing plate generally comprises a support such as a polyester film, the above photosensitive resin composition provided on the support, and a slip layer or a protective layer for the purpose of preventing adhesion with a negative film or a UV shielding layer comprising an infrared-susceptible substance which can be ablated by an infrared laser provided on the photosensitive resin composition. Making a flexographic printing plate from such a photosensitive resin plate material for a flexographic printing plate generally includes first subjecting the whole surface to ultraviolet exposure through the support (back exposure) to provide an even thin cured layer (floor layer), then directly subjecting the surface of the photosensitive resin layer to image exposure (relief exposure) through a negative film or from above the UV shielding layer in which a photomask has been formed by an infrared laser, and washing an unexposed part with a developing solvent or thermally melting the unexposed part followed by absorbing and removing it with an absorption layer followed by post-processing exposure.
General printing using a photosensitive resin plate for flexographic printing is performed by supplying, using an ink supply roll or the like, an ink to a projected surface of the recessed and projected resin plate and then bringing the resin plate into contact with an object to be printed to transfer the ink on the projected surface to the object to be printed. The ink for such general flexographic printing includes an aqueous ink, an emulsion ink, and an organic solvent-based ink, for example, a UV-curable ink or an ink using a vegetable oil or light naphtha.
In such flexographic printing that uses an organic solvent for development at the time of platemaking, problems of a photosensitive resin composition were that because of insufficient resistance to an organic solvent-based ink, the printing plate was destroyed during a long-time printing, or the printing plate was swelled and deformed to cause printing of a shape different from the original pattern required for the printing.
With respect to the solvent resistance of such a photosensitive resin composition, several methods have been proposed to overcome the problems as described above.
In order to improve the resistance to UV curable printing inks, Patent Document 3 proposes a photosensitive resin composition comprising at least one elastomer-based binder, a photopolymerizable unsaturated monomer, and a photopolymerization initiator, and if needed, a dye, a pigment, a polymerization inhibitor, an oxidation inhibitor, and a light degradation inhibitor. As examples thereof, Examples in Patent Document 3 show photosensitive resin compositions comprising various binder mixtures of a plastic elastomer-based block copolymer and a different block copolymer, two photopolymerizable monomers, benzyl dimethyl ketal as a photopolymerization initiator, an oxidation inhibitor, and a dye.
Patent Document 4 proposes a photosensitive resin composition comprising an elastomer resin, a hydrogenated terpene resin having a weight average molecular weight of 2000 or less, an amount of unsaturated bonds of 0.5 mol/100 g or less, and a refractive index at 20° C. of 1.497 or more, a photopolymerizable unsaturated monomer, and a photopolymerization initiator, and if needed, a dye, a pigment, a polymerization inhibitor, an oxidation inhibitor, and a light degradation inhibitor. As examples thereof, Examples in Patent Document 4 show photosensitive resin compositions comprising a styrene-isoprene-styrene copolymer, a hydrogenated terpene resin, two photopolymerizable monomers, and benzyl dimethyl ketal as a photopolymerization initiator.
In order to improve the resistance to an ink comprising a solvent or the like, Patent Document 5 proposes a photosensitive resin composition comprising a butyl rubber having an SP value of less than 8.1, a monomer having an ethylenically unsaturated bond, a photopolymerization initiator, and a water-dispersible latex, and if needed, a dye, a pigment, a polymerization inhibitor, an oxidation inhibitor, and a light degradation inhibitor. As examples thereof, Examples in Patent Document 5 show photosensitive resin compositions comprising butyl rubber, polyisoprene rubber, carboxylated nitrile rubber, butadiene rubber, a water-dispersible latex/hydrophilic monomer mixture, three photopolymerizable monomers, and benzyl dimethyl ketal as a photopolymerization initiator.
Patent Document 6 describes a photosensitive resin composition comprising, for example, a styrene-isoprene-styrene block polymer, an ethylenically unsaturated compound obtained by subjecting a system in which a liquid polybutadiene having a hydroxy terminal, acrylic acid, a compound having a phenol structure, and a compound containing at least one of phosphorus, sulfur, nitrogen, and boron are blended to ester interchange at 80 to 90° C. for 4 hours, 1,6-hexanediol dimethacrylate, and a photopolymerization initiator.
Patent Document 7 describes a photosensitive resin composition comprising, for example, a polystyrene-polybutadiene-polystyrene type block copolymer, a conjugated diene-based ethylenic polymer produced from a hydroxy group-containing polybutadiene and acrylic acid, hexamethylene dimethacrylate, and a photopolymerization initiator, and an original plate for flexographic printing using the same.